480V Split phase / 400V Threephase (AUS)

[Tomu]

HI there,

First post here, I hope I can get out into the forum and provide some answers to you in the future.

I am an Electrician here In Australia and spent my career so far in Industrial.

Currently I have a job which is putting me to the test so I thought someone might be able to offer some advice to me.

The situation:
A customer who is a Hydroponic farmer currently has "2 Phase" "Split phase" (480V) Whichever you like to call it. Three phase isn't available as it's in a rural area and the power is coming in via a SWER line.

He has approx 12Kw of pumps on one of the phases + House. The other phase has the processing machinery (only runs 3 Days a week) and refrigeration,

He approached me to install a three phase generator with auto start on the farm.

I can see installing a 2 phase Genny will come up, But they are expensive compared to a three phase here as 2 phase isn't common at all these days.
So My question for the EE forum so I can proceed with how to design this is.

1 - Is having unbalanced Phases on a generator detrimental to the life of it (30-40kVA)

2- If each phase has to be balanced, How close can they be. ~10-15%

3- What is an acceptable startup current ( imagine 10 Pumps firing up at once)

4- If I need to balance the load out, As processing happens only three times per week, + 1 phase won't even be connected. What can I use- IE just go run a few big resistors to "waste" the power?
Please fell free to ask for more information!
Thanks

 

[jim hardy]

A customer who is a Hydroponic farmer currently has "2 Phase" "Split phase" (480V) Whichever you like to call it. Three phase isn't available as it's in a rural area and the power is coming in via a SWER line.

He has approx 12Kw of pumps on one of the phases + House. The other phase has the processing machinery (only runs 3 Days a week) and refrigeration,

SWER ?
Single Wire Earth Return (SWER) ?

i guess i don't understand the concept of two phases on as single wire.

 

[Tomu]

Hi,

Its a 3 wire system. The transformer is centre tapped.

like this.

https://upload.wikimedia.org/wikipedia/en/2/29/Split_phase2.png

 

[Averagesupernova]

So you want to run a 3 phase generator to supply a split-phase facility. I believe it is done frequently. Are you familiar with a zigzag or double delta connection?
http://www.marathonelectric.com/generators/docs/manuals/SB504.pdf

<Moderator's note: fixed link>

 

[Tomu]

I have never heard of either. But I can get a bit of an idea from the link you provided to me. Do you have any experience in doing this?

 

[Averagesupernova]

I have never made this connection. I have faith though. The problem you have will be the power derating. You need to size the generator considerably larger than you would for true 3 phase loads. Asking to start 12 HP worth of pumps with a generator is a tough deal to begin with. A good utility will start a 12.5 HP motor with ease but making your own power is always a compromise.

 

[Tomu]

I'm going to have to break into the circuit and split it with a few contactors and Timers from the PLC.

If I don't need the Potential difference of 480, So Can I use two out of the three phases from the generator? Or do they need to be balanced.

 

[Averagesupernova]

I would try to better balance it by using the double delta or zigzag. Of course these are not truly balanced but it is better than letting one phase idle. Voltage regulation may be an issue if you let a phase idle. Best to ask the manufacturer of the generator.

 

[jim hardy]

here's a hobbyist's description of doing the ziz-zag
http://www.anoldman.com/strange/re-wiring_a_three_phase_generator

 

[Tomu]

So here in Aus, I take it that wiring it like this will give me 240/480?

 

[Averagesupernova]

Wiring it like what?

 

[jim hardy]

So here in Aus, I take it that wiring it like this will give me 240/480?

it's exactly same as in the generator manual that Averagesupernova linked to

i never trust that anybody's wire numbering scheme is "standard"

"anoldman" gives a decent tutorial here on why it works
before undertaking it i'd make sure of wire numbers ,, observe his precaution below
http://www.anoldman.com/power_systems/understanding_how_generators_work

The double delta and high delta produce perfect voltage and phase for household use. They are also very difficult to wire if you don't know exactly which coil is which. Many exciter circuits will not work with them either. Worse of all, they won't work with a 6-wire generator. The final configuration is called a zig-zag, and is most commonly used when residential 120v/240v is needed from a 3-phase generator:
...
The hardest part of re-configuring a generator like we've seen above is figuring out which of the wires coming out of the generator head are connected to which coils. Without a manufacturer's documents, your best bet is determining how the generator is currently wired and working backwards from there.

i've never done it , just stumbled across this hobbyist site and thought "That makes sense! "

old jim

 

[Baluncore]

You now have SWER. It is isolated with the transformer to 480VAC split with a centre tap. That provides a neutral and two opposite phases of 240VAC. You can run single phase 240VAC equipment between the neutral and one side of the transformer. You only need both sides if you need more power than is available from one side, or you run a big "two-phase welder". There should be no need to balance the two phases unless power use is above 50%.

A three phase generator with a neutral (earthed) connection obeys the same rules of load sharing. You need to allocate load to three active phases, but do not need to balance load unless you exceed 33% per phase. That does not mean you should not balance when you can. You should not waste energy with dummy loads.

The question then becomes, how do you select subsets of circuits that can be allocated to either a 2 or 3 phase system. The best answer is probably that the change over circuit will switch up to six load circuits. When on the SWER the contactors will balance 3 circuits to each of the 2 phases. When on the 3PH alternator the contactors will allocate 2 circuits to each of 3 available phases. You can use a pair of 3PH contactors to do the changeover.

P.S. For about a decade now, Australia has been standardised on 230VAC, but the voltage does not seem to have changed at my end yet.